Verify — contract-drift checks

Attach a responseSchema to an endpoint, run with verify, and mockr validates every served response body against it — reporting any mismatch. Run it against the real backend (via proxy / ctx.forward()) to catch when the API drifts from your declared contract, or against your mocks to catch when the mocks drift.

Declare a contract

mockGroup<Endpoints>()
  .get('/api/projects/:projectId/stats/', {
    responseSchema: z.object({ project_id: z.string(), progress: z.number() }),
    fn: (req) => ({ project_id: req.params.projectId, progress: 87 }),
  })
  .done();

responseSchema is any safeParse schema. It's independent of the response's TypeScript type — typically the real BE's full contract.

Turn on verification

await mockr({
  verify: true,                 // or run with the --verify CLI flag
  onDrift: ({ url, method, issues }) => {
    // e.g. fail CI, collect a report
    console.error('drift', method, url, issues);
  },
  groups: [mocks],
});

Option	Description
verify	boolean. Also enabled by --verify on the CLI.
onDrift	(info: { url, method, issues }) => void. Called once per response whose body fails its responseSchema.

When verify is on, every served body that has a matching responseSchema is checked. Failures call onDrift and log a drift line; the response is still sent. Endpoints without a responseSchema, and raw responses, are skipped.

Checking the real backend

Point a route at the upstream — proxy passthrough, or a handler that returns ctx.forward() — and keep its responseSchema. The served body is then the real response, so verify reports when upstream no longer matches your contract:

npx tsx serverMocker.ts --verify